Jailer: added the `-h` flag as a shorthand for `--help`

[seafoodfry]

Changes

Jailer: added the -h flag as a shorthand for --help.

Reason

This is a common convention - thought it would be useful.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check CONTRIBUTING.md.

PR Checklist

• [ ] If a specific issue led to this PR, this PR closes the issue.
• [x] The description of changes is clear and encompassing.
• [ ] Any required documentation changes (code and docs) are included in this PR.
• [ ] API changes follow the Runbook for Firecracker API changes.
• [ ] User-facing changes are mentioned in CHANGELOG.md.
• [ ] All added/changed functionality is tested.
• [ ] New TODOs link to an issue.
• [x] Commits meet contribution quality standards.

---

• [x] This functionality cannot be added in rust-vmm.

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Additional Details

Tried to replicate the workings of the ArgParser by starting from https://github.com/firecracker-microvm/firecracker/blob/e67b2bedbe01a958cc3140f0e951bd5ffa98ff0a/src/jailer/src/main.rs#L318-L320

Which leads to https://github.com/firecracker-microvm/firecracker/blob/e67b2bedbe01a958cc3140f0e951bd5ffa98ff0a/src/utils/src/arg_parser.rs#L320-L323

From there I tried to replicate the logic with this small script to see if this patch would work:

use std::env;                                                           
                                                                        
const ARG_SEPARATOR: &str = "--";                                       
const HELP_ARG: &str = "--help";                                        
const SHORT_HELP_ARG: &str = "-h";                                      
                                                                        
fn split_args(args: &[String]) -> (&[String], &[String]) {              
    if let Some(index) = args.iter().position(|arg| arg == ARG_SEPARATOR) {
        return (&args[..index], &args[index + 1..]);                    
    }                                                                   
                                                                        
    (args, &[])                                                         
}                                                                       
                                                                        
fn parse(args: &[String]) {                                             
    // Skipping the first element of `args` as it is the name of the binary.
    let (args, _) = split_args(&args[1..]);                             
                                                                        
    if args.contains(&HELP_ARG.to_string()) || args.contains(&SHORT_HELP_ARG.to_string()) {
        println!("found help args: {args:?}");                          
    }                                                                   
}                                                                       
                                                                        
fn main() {                                                             
    let args: Vec<String> = env::args().collect();                      
                                                                        
    parse(&args)                                                        
}

I tried it out and saw the following:

➜  ~/src/temp/args_help git:(master) ✗ ./target/debug/args_help --help 
found help args: ["--help"]
➜  ~/src/temp/args_help git:(master) ✗ ./target/debug/args_help -h    
found help args: ["-h"]
➜  ~/src/temp/args_help git:(master) ✗ ./target/debug/args_help --helpo
➜  ~/src/temp/args_help git:(master) ✗ ./target/debug/args_help --h    
➜  ~/src/temp/args_help git:(master) ✗ ./target/debug/args_help -hello

So it seems like this "should" work.